The head of Iran's elite Quds force is reportedly visiting Syria to advise the regime on repressing protests and the armed resistance, as consternation grew in Western capitals yesterday (Thursday) about Iranian and Russian meddling in the crisis.

Members of the opposition Syrian National Council said they had reliable intelligence that Qassem Suleimani was intimately involved with President Bashar al-Assad and his ruling coterie.

"It is his second visit at least," said Radwan Ziahdeh, an executive member of the council. "The Quds force is working mainly with training, helping militias and snipers."

William Hague, Britain's Foreign Secretary, told his Russian counterpart, Sergei Lavrov, of his grave concern that Russia continues to sell arms to the government. In a testy phone call that followed Mr Lavrov's inconclusive diplomatic mission to Syria earlier this week, the Russian replied simply that there was "nothing illegal" about the sales.

The Quds, or Jerusalem, brigade, is a special unit of the Islamic Revolutionary Guard Corps responsible for external relations that reports directly to Supreme Leader Ayatollah Ali Khamenei. Western and Arab experts and diplomats estimate that the number of troops and advisers from the Quds force in Syria to be in the high hundreds or low thousands. They have set up at least one base in Zabadani near the capital Damascus.

A Foreign Office spokesman said: "We are deeply concerned by credible information that Iran is providing equipment and technical advice to help the Syrian regime quash protests in Syria. Such support is unacceptable." Help includes riot control equipment and technical advice on "how to quash dissent and how to flood areas with security forces".

Iran is also said to be providing support to improve the regime's intelligence gathering and monitoring of protesters' use of the Internet and mobile phone network, including text messaging.

Opposition fighters of the Free Syrian Army claim to have captured 29 Iranians during the uprising and last week posted a video of five captives with their passports. Reports in the Arab media have claimed that snipers from Iranian-backed Hizbollah forces have been brought in from Lebanon to support government forces fighting the FSA.

The rebels yesterday appealed for the U.S. to supply weapons, rocket launchers, body armour, night-vision goggles and other equipment. Identifying himself only as Mohammed, one rebel spoke via the Internet to experts and journalists at a Washington think tank.

"The major point is logistical material support. We can do this ourselves, we're not asking for any troops," he said.

Faced with Russian and Chinese obstruction of a United Nations resolution, the international community has injected new urgency into its efforts to force the regime to stop the killing.

A State Department official has said that time was running out before the international community would have to "militarise" the situation, which would involve arming rebels or military protection for humanitarian aid.

Britain and other European states have temporarily withdrawn their ambassadors to Syria, while Germany has expelled four members of the Syrian embassy in Berlin for spying on regime opponents.